Sharp MZ

I've also tried removing the processor and get exactly the same. Seems like something more fundamental perhaps?

No, when you remove the ROM it doesn't end with that kind of multicolored checkboard : I never saw that on a working MZ-700. It is not an issue with ROM. If you remove CPU, I also believe you won't get that. So if you have that issue with no ROM or CPU, there is definitely another issue elsewhere. If you remove the CGROM, I won't expect that checkboard too. I had the same checkboard whetever I tried to change the ROM, CPU or CGROM. Those potential defective chips were tried to another MZ-700 and they worked. Since the PSU blowed, I cannot lead more tests for now.

I see two potential issues: VRAM and LSI (the worst case) or some 74LSxx chips between CPU and VRAM.

I've also tried removing the Z80 processor and it's still the same. Does that suggest something more fundamental?

I have three working MZ-700s, and currently only the MONITOR ROM is reproducing the problem perfectly on all three. I haven't tried to remove the processor but it must be a possible case.
I think it's normal after a little analysis, for what it's worth: The CPU isn't running any program because the ROM doesn't exist, the VRAM isn't initialized so it contains random data and as the display works we have this random series on the screen.
If we remove the processor, nothing is executed in memory either, the VRAM is not initialized and as the display works, we have to find this random display on the screen too.
Now, I think it must be either the MONITOR ROM or the Processor, so....
After that it doesn't exclude the VRAM either, but it would certainly be with a changing display because the monitor initializes it at startup and writes the header and then flashes a slider, all this has to show up on the screen somehow and it shouldn't be frozen.
So that was my little analysis of the situation, maybe it's wrong... I look forward to your constructive comments.

Hi I have a MZ80K with the same problem.
A static random screen, the pattern is always the same on power or reset.
I reproduced the fault on another MZ80K by testing a RAM checker eprom that it did not work in the faulty machine.

The both MZ80K could not see the eprom,the eprom was too slow 350ns. a 200ns eprom work fine in the good MZ80K but not the faulty one.
It still produced the same screen, after about 5 second snow appeared on the screen indicating the CPU was running and accessing the video ram area out side the permitted blanking periods.
The CPU is outputting addresses to refresh the RAM.
The CPU can not see the ROM/EPROM and running random code.
The owner of the K board has changed the CPU, ROM, G-ROM, VID-RAM, and RAM, fault still persists, the same static random screen.
I think the problem is faulty data buffer/drivers.
Before I replace the IC50 on the K, I will check that /CS0 is working and the ROM is being turn on.

hlide wrote: ↑Fri Apr 03, 2020 2:44 pm No, when you remove the ROM it doesn't end with that kind of multicolored checkboard : I never saw that on a working MZ-700. It is not an issue with ROM. If you remove CPU, I also believe you won't get that. So if you have that issue with no ROM or CPU, there is definitely another issue elsewhere. If you remove the CGROM, I won't expect that checkboard too. I had the same checkboard whetever I tried to change the ROM, CPU or CGROM. Those potential defective chips were tried to another MZ-700 and they worked. Since the PSU blowed, I cannot lead more tests for now.

I see two potential issues: VRAM and LSI (the worst case) or some 74LSxx chips between CPU and VRAM.

LSI seems to be working at least partially (and let's hope it's ok... because as hlide says, it's the worst case). It's responsible for generating video signal and well... it's there, and it's stable with correct characters from CGROM. So let's rule it out for now. Seems like it is getting totally (not that) random video data though. It can be one of any RAM/ROM chips wreaking havoc on data bus. Or like you said any 74xx chips, or the CPU itself.

Now, stupid question... you said screen looks identical each time you switch machine on. Is the pattern every time *the same*? all the way to the single characters displayed? Or random, with similar checker board pattern, but different each time you turn it on?

Yes, that would be interesting to take several shoots of the checkboard pattern after each boot to compare them.

Not the ROM or CPU themselves in my case as they worked when plugged on another MZ-700. But if data bus is mistigated by something else the ROM and CPU then the CPU will be impacted in its execution for sure, hence no bell when booting.

It's quite a challenge to find the culprit remotely
AndyR:
Do you, by any chance, have logic analyzer/oscilloscope/something that could show you signals on some of those chips? Could help lots
Anyway. Do you have anything else you can plug CPU to to verify it's working?

It does not seem to be VRAM to be honest.. as it's gated behind LS245 buffers and should allow computer to al least beep...
Taking a look on data/address bus would be essential to be honest.

Hi everyone.
Just to update you, the display is absolutely identical everytime I switch on.
I swapped the CPU for one I had spare which I believe is working but the output is just the same.
It doesn't beep on startup which would support the theory it's not using the Monitor ROM program at all?
Gortho: No analyser equipment unfortunately.
Looking more and more like the Monitor ROM I think.